Drive Shaft

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A drive shaft, or if you prefer, a driveshaft, driving shaft, tailshaft (a term favored in the land of Australian English), propeller shaft (prop shaft), or the more classically named Cardan shaft (a nod to the polymath Girolamo Cardano), is a mechanical component with a singular, thankless job: transmitting power. It's a rotating tube that bridges the gap between other parts of a drivetrain that, due to distance or a need for independent movement, can't be bolted directly together. Think of it as the long-suffering intermediary in a dysfunctional mechanical family.

As carriers of torque, these shafts are perpetually subjected to the twisting forces of torsion and the internal strife of shear stress. This stress is the raw difference between the torque fed into one end and the load resisting at the other. Consequently, they must be engineered with enough structural integrity to withstand this constant abuse without adding excessive weight. More weight means more inertia, and more inertia is just more work for the engine that has to spin it in the first place. It's a tedious balancing act.

To accommodate the chaotic reality of a moving vehicle, where alignment and distance are rarely constant, drive shafts are not simple, rigid rods. They frequently incorporate one or more universal joints—the mechanical equivalent of a flexible wrist—or other compliant connectors like jaw couplings or rag joints. To handle changes in length, which happen every time the suspension articulates, they often feature a splined joint or a prismatic joint, allowing the shaft to telescope without tearing itself apart.

History

The term "driveshaft" clawed its way into the engineering lexicon sometime in the mid-19th century, an era of magnificent sideburns and mechanical ingenuity. In an 1861 patent reissue for a planing and matching machine, a certain Stover used the term to describe the belt-driven shaft that powered his contraption. Curiously, his original patent didn't bother with the term, suggesting it was an afterthought. Another early appearance can be found in the 1861 patent reissue for the Watkins and Bryson horse-drawn mowing machine, where it referred to the shaft that transferred the rotational energy from the wheels to the gear train operating the cutters.

It wasn't until the 1890s that the term began to resemble its modern usage. In 1891, a man named Battles described the shaft connecting the transmission to the driving trucks of his peculiar Climax locomotive as the drive shaft. In the same year, Stillman applied the term to the shaft linking the crankshaft to the rear axle of his shaft-driven bicycle, a noble but ultimately doomed attempt to replace the greasy chain. By 1899, Bukey was using it to describe the shaft in his Horse-Power machine, which used a universal joint to transmit power from the wheel. That same year, Clark's Marine Velocipede patent used the term for the gear-driven shaft that sent power through a universal joint to the propeller shaft. Finally, in 1903, Crompton used the term to describe the shaft between the transmission and the driven axle of his steam-powered Motor Vehicle.

The evolutionary leap to gasoline-powered cars was marked by the 1898 Renault Voiturette, which holds the distinction of being the first to use a drive shaft. Across the Atlantic, the Autocar company was the first American manufacturer to adopt the drive shaft in a gasoline car. That 1901 vehicle now sits in a collection at the Smithsonian, a silent testament to a solved engineering problem.

Automotive drive shaft

Vehicles

In the world of automobiles, the drive shaft is the backbone of power delivery. A longitudinal shaft is often employed to ferry power from an engine and transmission at one end of the vehicle to the other, just before it's distributed to the wheels. More commonly, a pair of shorter drive shafts, often called half-shafts, are used to transmit power from a central differential, transmission, or transaxle directly to the wheels.

Front-engine, rear-wheel drive

In the classic front-engined, rear-wheel drive configuration, a long drive shaft is essential to bridge the vehicle's length. Two primary systems have dominated this space: the torque tube, which encloses the shaft and uses a single universal joint, and the far more common Hotchkiss drive, which leaves the shaft exposed and relies on two or more universal joints to handle suspension movement. This latter system was famously patented by the automobile company Panhard et Levassor, leading to its designation as the Système Panhard.

Most vehicles of this type feature a clutch and gearbox (or transmission) bolted directly to the engine. The drive shaft then extends from the gearbox to a final drive unit located in the rear axle. In this setup, the drive shaft remains motionless when the vehicle is stationary. However, some vehicles, particularly sports cars obsessed with achieving a more favorable weight distribution—like the Chevrolet Corvette C5/C6/C7, Alfa Romeo Alfetta, and the Porsche 924/944/928 family—opt for a rear-mounted transaxle. In most of these non-Porsche designs, this places the clutch and transmission at the rear, meaning the drive shaft connecting them to the engine spins continuously as long as the engine is running, regardless of whether the car is in gear.

The Porsche 924/944/928 models, however, took a different approach. Their clutch is mounted at the back of the engine in a traditional bell housing. The drive shaft, protected inside a rigid, hollow torque tube, transmits power from the clutch output to the rear-mounted transaxle (a combined transmission and differential). This means the Porsche driveshaft only rotates when the rear wheels are turning, as the clutch can decouple it from the engine's crankshaft. This design reduces the rotational inertia the engine has to overcome during gear changes, allowing the engine to rev more freely when the driver depresses the clutch. The torque tube itself is rigidly fastened to both the engine's bell housing and the transaxle case, fixing their alignment and substantially mitigating the twisting forces of rear-wheel drive reaction torque.

A drive shaft that connects a rear differential to a rear wheel is often called a half-shaft. The name is disappointingly literal: two of them are required to form a complete rear axle.

Early automobiles, in their primitive stages, often relied on cruder mechanisms like chain drive or belt drive instead of a drive shaft. A few even experimented with electrical generators and motors, a prescient but impractical solution for the time.

Front-wheel drive

If you find yourself in a conversation with someone speaking British English, be aware that the term "drive shaft" is typically reserved for the transverse shafts that deliver power to the wheels, particularly the front ones. The long shaft connecting the gearbox to a rear differential is instead called a "propeller shaft" or "prop-shaft." A prop-shaft assembly is composed of the propeller shaft itself, a slip joint to allow for length changes, and at least one universal joint. In four-wheel drive and rear-wheel drive vehicles, where the engine and axles are physically separated, it is this propeller shaft that dutifully transmits the engine's propulsive force to the axles.

The automotive industry has settled on several common types of drive shafts:

One-piece drive shaft: The simplest form, a single tube.
Two-piece drive shaft: Used in longer vehicles to prevent vibration and clear chassis obstacles, supported by a central bearing.
Slip-in-tube drive shaft: A more recent innovation designed with crash safety in mind. It's engineered to compress or collapse upon impact, absorbing energy that would otherwise be transferred into the passenger cabin. Hence its more dramatic alias, the "collapsible drive shaft."

Four wheel and all-wheel drive

These systems are a logical, if more complex, evolution of the front-engine, rear-wheel drive layout. A new component, the transfer case, is inserted between the transmission and the final drives. This unit splits the engine's power, directing it to both the front and rear axles. It may also contain reduction gears for off-road use, a dog clutch, or a differential to manage speed differences between the axles. This necessitates at least two drive shafts: one running from the transfer case to each axle. In larger vehicles, the transfer case might be centrally mounted and driven by its own short drive shaft from the transmission. In a vehicle like a Land Rover, the drive shaft to the front axle is significantly shorter and operates at a more severe angle than the rear shaft. This presents a greater engineering challenge, often requiring a more sophisticated and robust type of universal joint to ensure reliability under stress.

Modern light cars equipped with all-wheel drive, such as those from Audi or the humble Fiat Panda, often use a system that more closely resembles a front-wheel drive architecture. The transmission and front axle final drive are integrated into a single unit alongside the engine. A single, long drive shaft then runs the length of the car to power the rear axle. This design is favored when the torque is primarily biased to the front wheels for more car-like handling, or when a manufacturer wants to produce both four-wheel drive and front-wheel drive versions of a model using a large number of shared components. It's a matter of efficiency, both in performance and production costs.

Research and development

The automotive industry also subjects drive shafts to rigorous examination at testing facilities, because finding a component's breaking point in a lab is preferable to finding it on a highway.

On an engine test stand, a drive shaft is used to transfer a specific speed or torque from an internal combustion engine to a dynamometer, a device designed to measure force and power.

During such tests, a "shaft guard" is typically employed at the connection points. This serves two purposes: to prevent accidental contact with the violently spinning shaft and to detect a shaft failure, ideally before it sends shrapnel across the room.

Similarly, at a transmission test stand, a drive shaft connects the prime mover (the power source) to the transmission being evaluated.

Symptoms of a bad drive shaft

An automotive drive shaft is generally expected to last around 120,000 km (75,000 miles), but like all things, it eventually tires of its existence. If a vehicle begins to exhibit any of the following signs, it's a clear indication that the drive shaft is failing and requires immediate attention.

Clicking or squeaking noise: A rhythmic clicking, squeaking, or grinding sound emanating from underneath the vehicle during motion is a classic symptom.
Clunking sounds: Sharp clunking noises, particularly when turning, accelerating, or shifting into reverse, suggest excessive play in the universal joints.
Vibration: An intense vibration felt through the floor of the vehicle is an early and common sign of a failing drive shaft. Worn-out couplings, u-joints, or support bearings can disrupt the shaft's balance, causing it to vibrate excessively.
Turning problems: Difficulty or strange resistance when turning the vehicle, at both low and high speeds, can be traced back to a defective drive shaft binding up.

Cardan shaft park brakes

A Cardan shaft park brake is a system that acts upon the drive shaft itself, rather than the wheels. These are typically found on small to medium-sized trucks. This design is notoriously prone to failure and has been implicated in numerous incidents where trucks have rolled away on slopes, prompting the issuance of formal safety alerts. Heavy vehicles with this type of brake usually have a large, ratchet-style handle, similar to a car's hand brake, as opposed to the air-powered button or lever found in modern heavy-duty trucks.

The risk factors for drivers are predictable: parking on a steep incline, especially when heavily loaded; not applying the brake with sufficient force; altering the vehicle's load or balance while parked on a slope; or parking where one side of the vehicle has poor traction and is able to slip. The use of wheel chocks is a low-tech but effective method for preventing a vehicle from making an unscheduled and catastrophic journey down a hill.

Motorcycle drive shafts

Drive shafts have been a feature on motorcycles for over a century, appearing on machines like the Belgian FN motorcycle from 1903 and the Stuart Turner Stellar motorcycle of 1912. As an alternative to the ubiquitous chain and belt drives, a shaft drive offers a clean, long-lasting, and relatively maintenance-free solution. However, this convenience comes at a cost. A significant disadvantage is the need for helical gearing or a spiral bevel gear to turn the power 90 degrees from the shaft to the rear wheel. This gearing inevitably introduces parasitic power loss.

BMW has been a steadfast producer of shaft-driven motorcycles since their first model in 1923. Moto Guzzi has been building their iconic shaft-drive V-twins since the 1960s. The British firm Triumph and the major Japanese manufacturers—Honda, Suzuki, Kawasaki, and Yamaha—have all produced shaft-drive models. Even the Lambretta motorscooters, from type A to type LD, were shaft-driven, as was the NSU Prima scooter.

Motorcycle engines where the crankshaft is oriented longitudinally, parallel to the frame, are particularly well-suited for shaft drive. This layout requires only a single 90-degree turn in the power flow, minimizing complexity and power loss compared to a transverse engine which would require two. Bikes from Moto Guzzi and BMW, as well as the Triumph Rocket III and the Honda ST series, all utilize this longitudinal engine layout.

Motorcycles with shaft drive are susceptible to a phenomenon known as shaft effect, where the rear chassis tends to rise or "climb" under acceleration. This is the opposite of the "squat" exhibited by chain-drive motorcycles. To counteract this undesirable behavior, manufacturers have developed sophisticated rear suspension systems, such as BMW's Paralever, Moto Guzzi's CARC, and Kawasaki's Tetra Lever.

Marine drive shafts

On a power-driven ship, the drive shaft, more commonly called the propeller shaft, forms the critical link between the internal driving machinery and the external propeller. It must pass through the ship's hull, requiring at least one shaft seal or stuffing box to prevent the ocean from inviting itself inside. The immense axial force generated by the propeller, known as thrust, is transmitted to the vessel's structure via a thrust block or thrust bearing. In all but the smallest boats, this bearing is integrated into the main engine or gearbox. These shafts can be fabricated from stainless steel or, in more modern applications, advanced composite materials, depending on the vessel's size and purpose.

The final portion of the drivetrain that connects directly to the propeller is known specifically as the tail shaft.

Locomotive drive shafts

The geared steam locomotives of the late 19th century, such as the Shay, Climax, and Heisler types, used drive shafts with quill drives to distribute power from a centrally mounted multi-cylinder engine to each of the pivoting trucks. On these rugged geared steam locomotives, one end of each drive shaft was coupled to the driven truck through a universal joint, while the other was powered by the crankshaft, transmission, or another truck via a second universal joint. The quill drive also allowed the shaft to slide telescopically, varying its length as the trucks rotated to navigate curves in the track.

Cardan shafts are also utilized in some diesel locomotives (primarily diesel-hydraulics like the British Rail Class 52) and certain electric locomotives (for instance, the British Rail Class 91). They are also widely employed in diesel multiple units.

Drive shafts in bicycles

The drive shaft has existed as an alternative to the chain-drive on bicycles for over a century, yet it has never managed to achieve widespread popularity. A shaft-driven bicycle (sometimes called an "Acatène," after an early manufacturer) presents a distinct set of advantages and disadvantages.

Advantages

The drive system is less susceptible to becoming jammed by foreign objects.
The rider avoids the inevitable grime of chain grease and the risk of injury from "chain bite," which occurs when clothing or a body part is caught between an unguarded chain and a sprocket.
Maintenance is significantly lower than a chain system, especially when the drive shaft is fully enclosed in a protective tube.
Performance is more consistent. Dynamic Bicycles, a company with a clear vested interest, claims a drive shaft bicycle can maintain 94% efficiency, whereas a chain-driven bike's efficiency can fluctuate between 75% and 97% depending on its condition. This claim, it should be noted, is begging for a citation.

Disadvantages

A drive shaft system is heavier than a chain system, typically adding 0.5–1 kg (1–2 lb) of weight.
Many of the supposed advantages, such as cleanliness and safety, can be achieved on a chain-driven bike simply by covering the chain and sprockets.
The use of lightweight derailleur gears with a wide range of ratios is not feasible, although internally geared hub gears can be used.
Removing the rear wheel can be more complicated in some designs, a problem it shares with some chain-driven bicycles that use hub gears.

PTO drive shafts

Drive shafts also serve to transfer power from an engine's Power Take-Off (PTO) unit to vehicle-mounted accessories, such as a large air compressor. A drive shaft is used in situations where there isn't enough space adjacent to the engine for the accessory. The shaft bridges the gap between the engine's PTO and the equipment, allowing the accessory to be mounted elsewhere on the vehicle's chassis.

Drive shaft production

The manufacturing process for drive shafts is evolving. The filament winding production process is gaining traction for the creation of lightweight, high-strength composite drive shafts. Several companies in the automotive industry are reportedly exploring how to adapt this technology for high-volume production, presumably to shave a few grams off their vehicles and a few milliseconds off their lap times.